Those Exciting Rhododendron Epidermal Appendages
Part II: Subgenus Rhododendron
Arthur P. Dome
Seattle, Washington

The Subgenus Rhododendron contains those plants that are considered "lepidote" or scaly. The species concerned here belong to the Section Pogonanthum and the Section Rhododendron . They have many interesting scales and some also have hairs on the underside of certain leaves.

I'm sure you were really intrigued the first time you looked at certain rhododendron leaves and then turned them over and saw something new and unexpected. What happened to the smooth green surface that was customary? With a second look it appeared that this unusual substance or growth was actually a part of the leaf. Depending upon the leaf, it could have been scales, bristles, hairs of various types, or a felty-type growth of varying degrees called "indumentum." In the case of the Subgenus Rhododendron they would always be scales and in some cases hairs also. John Macqueen Cowan (2) states: "These structures are minute organs, each of which is capable of performing definite functions and their real significance is in relation to the well-being of the plant. Together, in a large part, they regulate the water supply and thus enable the Rhododendron to withstand the extremes of climate they are called upon to endure. There can be no doubt that, without them, some rhododendrons would have failed in the habitats where they prevail."

These epidermal appendages on the leaves of rhododendrons have always been one of the determining factors in the classification and naming of these plants. Now newer developments in DNA technology and the use of chemical analysis comparisons in classifying and reclassifying other genera within the family Ericaceae is very important. Those of you who recently attended the Fifth International Rhododendron Species Symposium in Bellevue, Washington, had the opportunity to listen to the world's leading authorities discuss the newer DNA technology and other comparisons. However, it was determined that these epidermal appendages will still play an important role in classifying and identifying plants.

These eight photographs illustrate some of the different scales that one might find on the plants in Subgenus Rhododendron . Age and environment can affect the size and color of these appendages. In some cases there were discrepancies between references in identifying a specific scale or hair. In this case the reference with the latest publication date was favored.

R. anthopogon ssp. anthopogon , S. Pogonanthum . Young new leaf underside closely covered with brown lacerate scales.* Photo by Arthur P. Dome		R. auritum , S. Rhododendron , Ss. Tephropepla . Mature new leaf underside pale glaucous green with entire type scales. Photo by Arthur P. Dome

R. baileyi , S. Rhododendron , Ss. Baileya . Mature new leaf underside with numerous overlapping crenulate scales. Photo by Arthur P. Dome		R. edgeworthii , S. Rhododendron , Ss. Edgeworthia . Mature new leaf side view showing densely woolly loriform hairs that completely cover the entire type scales. Photo by Arthur P. Dome

R. genestierianum , S. Rhododendron , Ss. Genestieriana . Mature new leaf underside glaucous with papillae and two sizes of entire type scales. Photo by Arthur P. Dome		R. glaucophyllum ssp. glaucophyllum , S. Rhododendron , Ss. Glauca . Mature new leaf underside glaucous with papillae, vesicular and entire type scales. Photo by Arthur P. Dome

R. fragariflorum , S. Rhododendron , Ss. Fragariflora . Mature new leaf upperside with crenulate scales and the underside covered with entire type scales. Photo by Arthur P. Dome		R. lepidostyium , S. Rhododendron , Ss. Trichoclada . Mature new leaf underside glaucous with vesicular scales and loriform setose hairs. Photo by Arthur P. Dome

Acknowledgments
The author is indebted to Steve Hootman, Co-Director-Curator, Rhododendron Species Foundation, Federal Way, Washington, for proofreading the text of this article.

References
1. Chamberlain, David, and Roger Hyam, George Argent, Gillian Fairweather, Kerry S. Walter. 1996. The Genus Rhododendron, Its classification & synonymy . Edinburgh: Royal Botanic Garden Edinburgh.
2. Cowan, John Macqueen. 1950. The Rhododendron Leaf . Edinburgh: Oliver and Boyd.
3. Davidian, H. H. 1982. The Rhododendron Species, Vol. I. Lepidotes . Portland, OR: Timber Press.
4. Cullen, J. 1980. Notes from the RBGE, Vol. 39, No. I . Edinburgh: Royal Botanic Garden Edinburgh.

* S = Section, Ss = Subsection, ssp = Subspecies, var. = Variety, syn.= synonym.

Arthur Dome's Part I of this series appeared in the Fall 1999 issue of the Journal. In photographing the leaves in this series, Dome has used plants growing at the Rhododendron Species Botanical Garden. Magnification varies from x10 to x50.